Eric Dinerstein is a rhino expert with the international conservation organization WWF and author of Return of the Unicorns, an analysis of Indian rhino conservation efforts.

Dinerstein explained that the rhinos have a high rate of natural mortality.

Calves often fall victim to tigers or are separated from their mothers by monsoon floods. Males engage in vicious battles during mating season that frequently end in the loser's death.

Still, Dinerstein suspects that there is a human hand behind the Babai Valley mystery.

"The likely answer is that many of them were poached," he said.

"The truth is that even back in 1975 I was told [by local people] not to go into the Babai Valley, because there were a lot of poachers there and it was very rough. There has been poaching there since long before there the [Maoist] insurgency," he said.

As for the missing carcasses, Dinerstein suggests that many factors could have caused them to disappear.

He's seen local Nepalese carry off the remains of a rhino that died naturally.

"Every part of the animal was considered valuable," he said. "There wasn't a shred of that rhino left."

The reserve's animal denizens may have also played a part.

"There are lots of scavengers on the [southern plain known as the] Teraimammals, birds, lots of species," Dinerstein said.

"If somebody had been patrolling they would have been tipped off [to a dead rhino] by lots of vultures.

"But because nobody was patrolling, you could easily have a carcass disappear quite quickly without much of a trace."